1. Field of the Invention
Embodiments of the invention generally relate to a safety valve. More particularly, embodiments of the invention relate to a safe mode indication device for a safety valve actuator.
2. Description of the Related Art
Various designs of valve actuators exist that operate to open and close valves in a variety of uses. The petroleum industry utilizes these actuators to operate gate valves that incorporate a sliding gate within a valve body to selectively block fluid flow through tubing. Positioning gate valves along tubing at various locations controls and directs the flow of fluids through the tubing. An assembly known as a christmas tree includes these valves along with spools, pressure gauges, fittings, and/or chokes connected to the top of a well in order to direct and control the flow of formation fluids or production fluids from the well.
In operation, a valve stem extending from the valve body of the gate valve moves axially within the valve body in order to move the sliding gate between an open position and a closed position. A shaft within an actuator for the gate valve engages the valve stem to impart the axial movement to the valve stem. Typically, the actuator includes a spring to bias the shaft within the actuator such that the gate valve provides a fail-safe to the closed position. In this manner, force applied to the shaft of the actuator from either a hydraulic, pneumatic, or mechanical source, depending on the type of actuator, overcomes the bias of the spring to move the sliding gate to the open position. Other gate valve designs provide for the fail-safe in the open position or a fail-in-position which maintains the position of the sliding gate upon failure. If the shaft or a top shaft coupled to the shaft extends external to the actuator, the amount of the shaft extending from the actuator may serve as a visual indication as to whether the valve is in the open position or the closed position.
The actuators commonly used to open and close the gate valves include mechanical overrides that use a mechanical force, automatic actuators such as various designs of pneumatic or hydraulic actuators, or combination actuators having both a mechanical override and an automatic actuator. Since most automatic operations of the actuators have a maximum capability for applying force to the valve stem, the combination actuator permits additional opening/closing power on a temporary basis without having to remove the original automatic actuator. Inadvertent loss of pneumatic or hydraulic pressure to the automatic actuator closes the gate valve which can interrupt production and interfere with wellbore completion operations occurring through the valve. The combination actuator provides a back-up for automatic operation, allows for testing, and enables an operator to lock the valve in the open position using a handwheel assembly of the mechanical override during various wellbore completion operations.
The combination actuators described above, however, fail to provide a visual indication whether the valve has been actuated using the mechanical override, the automatic actuation, or both. Although the shaft extending from the valve may in some cases provide a visual indication that the valve is either open or closed, the visual indication does not communicate which actuation mechanism is being used to position the valve in the open or closed position or whether the valve is fully open or fully closed. The lack of this communication can be detrimental in various situations. For example, a valve operator may believe that a valve having a fail-safe closed mechanism has been automatically actuated into an open position, when in fact the valve may have been opened using the mechanical override. Since the mechanical override may also override the fail-safe closed mechanism, the operator is thus under the mistaken belief that the valve is operating in a safe mode. In the event of a shut-down, the valve will remain locked open and be prevented from failing or moving into the closed position. In another example, when using the mechanical override, it is critical that the handwheel assembly is completely rotated until the valve is fully open or fully closed. Only a few turns of the handwheel can offset the valve gate, and yet the appearance of the handwheel shaft extending from the actuator cannot communicate this slight offset. Thus when automatic operation occurs, the valve cannot be situated in a fully open or fully closed position, potentially causing leakage in the valve and un-even wear of the valve's seat and/or seals of the gate or body by fluid flow through the slightly offset valve.
Thus, there exists a need for an improved mechanical override for use with an actuator for a valve. There exists a further need for a valve having a mechanical override that provides a visual indication of the status and/or position of the valve, as well as which actuation mechanism is operating the valve, when the valve is operating in a safe mode, and/or when the mechanical override is intervening with the fail-safe mechanism.